Clearly the absolute number of casualties (not even civilian casualties in this case, that is allegedly more like 15k) is not sufficient on its own to define a genocide.
For example, look at the section for Czechoslovakia (selecting it since it's inarguable Germans had lived in the country formerly known as Bohemia for centuries) alone:
> Between 700,000 and 800,000 Germans were affected by irregular expulsions between May and August 1945.[108] The expulsions were encouraged by Czechoslovak politicians and were generally executed by order of local authorities, mostly by groups of armed volunteers and the army.[109] [...] Transfers of population under the Potsdam agreements lasted from January until October 1946. 1.9 million ethnic Germans were expelled to the American zone, part of what would become West Germany. [...] More than 1 million were expelled to the Soviet zone, which later became East Germany.[110] The West German government estimated the expulsion death toll at 273,000 civilians,[115] and this figure is cited in historical literature.[116] However, in 1995, research by a joint German and Czech commission of historians found that the previous demographic estimates of 220,000 to 270,000 deaths to be overstated and based on faulty information. They concluded that the death toll was between 15,000 and 30,000 dead, assuming that not all deaths were reported.[117][118][119][120]
It's just that 'forced population transfers' were historically considered not outside the bounds of propriety.